<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            margin: 0 auto;
            border: 1px solid red;
            width: 100px;
            height: 100px;
            font-size: 50px;
            text-align: center;
            line-height: 100px;
            background-color: pink;
        }

        button {
            margin: 0 auto;
            /* background-color: #663399; */
            display: block;
            width: 50px;
            height: 50px;
        }
    </style>
</head>

<body>
    <div class="box">
        <span>10</span>
    </div>
    <button>开始</button>
    <script>

        // 2. 倒计时
        // - 三个按钮，先实现基本功能，不用控制按钮的禁用
        var button = document.getElementsByTagName('button')[0]
        var span = document.getElementsByTagName('span')[0]
        var div = document.getElementsByTagName('div')[0]

        var t
        console.log(button, span, div)
        button.onclick = function () {
            var btntext = button.innerText
            if (btntext == '开始') {
                button.innerText = '暂停'
                // clearInterval(t)
                t = setInterval(() => {
                    span.innerText -= 1
                    div.style.backgroundColor = randomColor()
                    if (span.innerText == 0) {
                        clearInterval(t)
                        button.innerText = '重置'
                    }
                }, 100);
            } else if (btntext == '暂停') {
                button.innerText = '继续'
                clearInterval(t)
            } else if (btntext == '继续') {
                button.innerText = '暂停'
                t = setInterval(() => {
                    span.innerText -= 1
                    div.style.backgroundColor = randomColor()
                    if (span.innerText == 0) {
                        clearInterval(t)
                        button.innerText = '重置'
                    }
                }, 100);
            } else if (btntext = '重置') {
                span.innerText = 10
                button.innerText = '开始'
            }
        }



        function randomColor() {
            var r = random(0, 255).toString(16);
            var g = random(0, 255).toString(16);
            var b = random(0, 255).toString(16);
            return "#" + addZero(r) + addZero(g) + addZero(b);
        }
        function random(max, min) {
            return Math.round(Math.random() * (max - min) + min);
        }
        function addZero(n) {
            return n.length < 2 ? "0" + n : n;
        }
    </script>
</body>

</html>